The Shape of the Milky Way's Dark Halo

Abstract

The distribution of shapes of galaxies' dark halos provides a basic test for models of galaxy formation. To-date, few dark halo shapes have been measured, and the results of different methods appear contradictory. Here, we add to the sample of measured shapes by calculating the flattening of the Milky Way's dark halo based on the manner in which the gas layer in the Galaxy flares with radius. We also test the validity of this technique -- which has already been applied to several other galaxies -- by comparing the inferred halo flattening to that obtained from a stellar-kinematic analysis, which can only be applied to the Milky Way. Both methods return consistent values for the shape of the Milky Way's halo, with a shortest-to-longest axis ratio for the dark matter distribution of q = 0.75 +/- 0.25. However, this consistency is only achieved if we adopt a value of R_0 = 7 +/- 1 kpc for the Sun's distance to the Galactic center. Although this value is smaller than the IAU-sanctioned R_0 = 8.5 kpc, it is quite consistent with current observations. Whatever value of R_0 is adopted, neither method returns halo parameters consistent with a disk-like mass distribution, for which q ~ 0.2. This finding rules out cold molecular gas and decaying massive neutrinos as dark matter candidates.